Screw or helical compressors are commonly used in air conditioning applications to compress refrigerant as part of the refrigeration cycle. Screw compressors are composed of meshing screw or helical rotors. While two rotor configurations are the most common design, screw compressors are also known in the art having three, or more, rotors housed in respective overlapping bores so as to co-act in pairs. The rotors of a typical screw compressor are mounted in bearings at each end in housing end plates at the inlet and discharge side. Refrigerant is compressed by the screw rotors toward the discharge side and discharged through ports and into a discharge line.
In normal applications, a solution or mixture of oil and refrigerant is used for lubricating screw compressor bearings and rotors. This lubricant becomes entrained in the refrigerant while the refrigerant passes through and is compressed. If this entrained lubricant is not separated and recovered by some means, it passes through condenser and liquid line and accumulates in the evaporator where it is mixed with liquid refrigerant. As a result, evaporator heat transfer effectiveness is degraded. Oil foam may also be created, which is entrained in suction flow entering the compressor, reducing the refrigerant flow rate of the compressor. Even worse, lubricant supply for bearing and rotor lubrication is eventually depleted.
In the past, oil separators have been utilized immediately downstream of the compressor. While oil separators do separate the lubricant, they have not always provided fully satisfactory results. As an example, the lubricant removed with such a separator will be at a high pressure, and may have an appreciable amount of refrigerant mixed in with the oil. This lowers its viscosity, degrading its usefulness as a bearing lubricant. The use of a separator can also cause a pressure drop in the compressed refrigerant, which is undesirable. A separator may also radiate sound due to internal pressure pulsations acting on its walls. A separator may also add considerable cost to the system since it is a pressure vessel of considerable size.
Another approach to lubricant separation is by use of a concentrator, or still, attached to the evaporator, also sometimes referred to as a generator as, shown for example in U.S. Pat. No. 6,182,467 B1. In such systems, a portion of the oil and refrigerant mix residing in the evaporator is made to flow into the concentrator, where means are provided for heating the mixture to cause some liquid refrigerant to vaporize. The liquid remaining thereby contains a higher fraction of oil. By suitable choice of the amount of refrigerant vaporized, a liquid with sufficient viscosity for use as a bearing lubricant may be created.
Referring to FIG. 3, such a prior art lubricant still is shown in detail, wherein the still 28 comprises a pressure tight vessel 30, which includes an inlet 32 for oil laden refrigerant 7, drawn off the evaporator, below the liquid level line, an outlet 34 for gaseous refrigerant, an oil outlet 36 for out flowing concentrated oil that has undergone separation. Still/reservoir 28 further includes a coil 42 through which the hot refrigerant flows for transfer of heat to the incoming oil/refrigerant mixture. Coil 42 has an inlet 38 for hot refrigerant and an outlet 40 for cooled refrigerant having gone through a heat transfer process.
The use of such stills for creating a lubricant from the oily refrigerant mix normally residing in an evaporator is a known art, with the viscosity resulting from still action in the range of 3 to 20 centipoise (cP). However, for some screw compressors, particularly those operated at low speeds, much higher lubricant viscosity of at least 50 cP is required. Approaches to achieving higher viscosity lubricants in conventional stills are less than satisfactory. For example, simply increasing the amount of heat provided to vaporize refrigerant may result in somewhat higher lubricant viscosity but may also incur a severe penalty to system efficiency since the extra heat provided must be accounted for when calculating the system efficiency. Moreover, conventional stills are defective in producing adequate lubricant viscosity during operating transients that result in sudden increases in the influx rate of oil laden refrigerant from the evaporator. During such transients the entering liquid tends to flood the still, mixing with liquid that has resided in the still for some time and lowering its viscosity.